Transplantation of hematopoietic stem cells from an allogeneic donor can be followed by serious graft-versus-host disease (GVHD) despite the best available regimen for immune suppression. Experimental animal data and results of clinical trials have demonstrated that T cells play a central role in GVHD. Recent advances in understanding the mechanisms of peripheral T cell tolerance have made it feasible to test the hypothesis that GVHD can be prevented in man by T cell receptor (TCR) partial agonist ligands through the selective depletion of alloreactive T cells. Transplantation tolerance is facilitated by activation-induced cell death (AICD) of peripheral T cells triggered by the specific alloantigens, and TCR signals are indispensable for induction of antigen-specific tolerance. TCR engagement can induce either T cell proliferation and differentiation or AICD. Monoclonal antibodies (mAb) to CD3, a monomorphic chain associated to the TCR complex, mimic antigen and induce TCR signaling and T cell activation. Fc receptor (FcR)-binding anti-CD3 mAbs recruit antigen-presenting cells (APC) and deliver full agonist signals that promote T cell proliferation and effector functions, independent of antigen. In contrast, non-FcR-binding anti-CD3 mAbs induce a partial agonist TCR signaling pattern causing anergy in naive T cells and AICD in antigen-activated, cycling T cells. In mice, we found that non-FcR-binding anti-CD3 mAbs induce selective depletion of donor T cells that recognize recipient alloantigens thereby preventing GVHD across MHC disparities, and spare T cell specific to third party antigens. In collaboration with J. Tso, we designed a `humanized'non-FcR-binding anti-CD3 mAb, visilizumab, which delivers a partial agonist signal to the TCR. Visilizumab is more effective than other anti-CD3 mAbs in inducing AICD of activated, cycling human T cells. Our preliminary results in human trials show that visilizumab can induce complete clinical responses in some patients with severe acute GVHD, refractory to conventional immune suppressive agents. We propose here to study efficacy of visilizumab in the prevention of GVHD after hematopoietic cell transplantation (HCT) from HLA incompatible donors, and assess whether its immune suppressive effects are selective for alloantigens. We will pursue the following specific aims: Aim 1: Determine the safety and efficacy of humanized non-FcR-binding anti-CD3 mAb visilizumab in preventing severe GVHD after transplantation of T-replete hematopoietic cell grafts from HLA-incompatible donors. Aim 2: Investigate whether visilizumab therapy depletes the alloreactive T cell pool, while sparing immunity to third party alloantigens, cytomegalovirus (CMV) and Epstein-Barr virus (EBV), and tumor- associated antigen WT1. PUBLIC HEALTH RELEVANCE: The development of effective regimens for the prevention of GVHD will improve patient safety and survival after transplantation, and extend the use of this procedure to a larger number of patients. Progress in the prevention of GVHD has the potential for advancing the fields of organ transplantation and therapy for autoimmune disorders. Anti-CD3 mAbs represent the prototype of a new class of therapeutic agents that suppress the immune system by inducing an abortive activation and T cell death. What makes anti-CD3 mAbs unique and more attractive than other agents is the potential for achieving immunological tolerance rapidly and irreversibly. Showing that non-FcR-binding anti-CD3 mAbs spare T cells specific for viral and tumor-associated antigens is key to predict patient safety.